Process for electroplating steel wires and coated wires thus produced

ABSTRACT

Steel wire are electroplated with layers of copper/lead, then zinc, then nickel, followed by solid state diffusion of the layers by Joule effect.

The present invention relates to a process for electroplating steelwires as well as to the products thus obtained, which products can beemployed for the manufacture of reinforcing structures for elastomericmaterial articles, and more particularly for tires.

More particularly, this invention relates to a process for coating steelwires with a thin layer of an alloy, containing copper, zinc, nickel andlead, by electroplating and subsequent diffusion by Joule effect.

BACKGROUND OF THE INVENTION

In the production of reinforced elastomeric materials, in particular ofradial tires, wire cords have been employed up to the present time, asis well known, which cords are made up of steel wires coated byelectroplating with brass, preferably of a composition betweenCu/Zn=60/40 and Cu/Zn=70/30 (% by weight) at the amounts between 2 and 8g/Kg of steel and of 1-2 μm thickness for a wire diameter between 0.75and 1.40 mm. Such wires ar then wet-drawn according to a chiplessprocedure and employed for the production of said cords.

It it also well known that such cords, in addition to a high ultimatetensile stress and to a high flexibility, also show, during the curingprocess with a standard mix, an adhesion to rubber (or other elastomericmaterial) which is much larger than the value of adhesion that can beascribed to the mere friction effect. At the present time such propertyis considered to stem substantially from the formation and growth offilms of some hundred Å thickness of copper and zinc sulfides at therubber/brass interface, as a consequence of the chemical reactionsoccurring during vulcanization.

On the other hand, it has been observed that the adhesion between steeland the coating is also of the same importance. Thus it is clearlyevident that any physical or chemical phenomenon, such as for instancecorrosion, which alters the state of surfaces, affects thecord/elastomer adhesion negatively.

Such drawback manifests itself frequently, because rubber mixescommercially employed in the production of tires are hardly everanhydrous, and give rise during vulcanization to the formation of a ZnOfilm, in addition to sulfides films, and next, owing to material aging,to the formation of hydroxides with the consequent definitive separationof the cord from the rubber. This was shown by a prolonged treatmentwith overheated steam (120° C. or more), in the presence of chlorides.

In that case the examination of the state of surfaces put into evidencethe presence of rust at the interface between steel and coating.

On the other hand, it is well known that car and truck tires aresubjected during employment to severe mechanical stresses, especially atthe highest speeds or in off-road runs.

As the safety of passengers and of load depends on tire reliability, itis necessary that the cord/elastomer adhesion be particularly strong,not only at the very beginning, but also during the full tire life.

SUMMARY OF THE INVENTION

Thus it is a main object of the present invention to provide a coatingfor steel wires that results in an improved starting adhesioncord/elastomer and that is stable in time, as a result of the increasein the corrosion resistance. It is another object to provide a coat fora wire which permits mechanical working of the wire and drawing of thewire without producing chips, and with no remarkable changes in thesurface composition and in the continuity of the coating itself.

In accordance with the present invention there is provided a coatingmade up of copper, zinc, nickel and lead electrodeposited as successivelayers on steel wires and then transformed into an alloy through solidstate diffusion a Joule effect.

It is worthwhile observing that the present invention is not limited tothe selection of the components mentioned above and of their percentageamounts in the alloy, but, in the formation of said coating, the presentinvention also sets forth the order and the manner in which theconstituents of the alloy are to be plated onto the steel support.Namely, copper and lead are electrodeposited simultaneously in a firststep, zinc is electrodeposited in the next step and then nickel iselectrodeposited.

In such a way a four-component coating is obtained which, in addition tobe unprecedented in the technical literature, also shows a very highcorrosion resistance due to the presence of nickel, as well as a veryhigh mechanical workability, which is ensured by the presence of lead.

The adhesion between said coating and the rubber, which adhesion in thecase of the known brass-coated wires stems from the formation of copperand zinc sulfides films, is increased in the coating according to thepresent invention by the further formation of a nickel sulfide film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the concentration of lead in the coating as afunction of the concentration of lead in the bath.

FIG. 2 is a graph showing the analysis of the surface of a coating.

DETAILED DESCRIPTION OF THE INVENTION

The production of steel wires coated with a thin layer of the alloyaccording to the present invention can be carried out in any plantdesigned for continuous traditional type working processes, bothpreliminary and successive to the electroplating operation.

More particularly, steel wires of diameters between 0.75 and 1.4 mm,obtained by dry-drawing of a steel rod of 5.5 mm diameter are subjectedbefore the electroplating process to a heat treatment carried out atabout 1,000° C. in a furnace, and to a patenting process at atemperature between 500° and 600° C. After such treatment, the wiresurface is drastically cleaned by passing it through a series of tankscontaining 2M H₂ SO₄ at 35° C., in which said wire takes on in turnpositive and negative polarities with current densities between 700 and800 A/dm². After such treatment and a suitable washing of the wiresurface, such surface is brilliant and free from impurities and oxides,as can be proved by a check under a metallographic microscope.

As regards the final treatment, the wire coated according to the processof the present invention is cleaned by removing the surface oxides bymeans of a washing operation with 10% H₃ PO₄ at 40° C. and then withwater. Hot air drying is then performed, followed by winding of the wireand the transfer of the same to the drawing and stranding departments.

The basic feature of the present invention lies thus in the way ofcarrying out the electrodeposition operations of the above-mentionedalloy components on the steel wire support and of performing thediffusion process by Joule effect.

More specifically, the present invention concerns a process for theproduction of electroplated steel wires, intended for the manufacture ofreinforcing structures for elastomeric material articles, especially fortires, said process being characterized by the following succession ofoperations:

(a) simultaneously electrodepositing copper and lead from an alloy bathon a steel wire, which has been previously subjected to theabove-mentioned preliminary treatment,

(b) electrodepositing zinc from a zinc plating bath,

(c) electrodepositing nickel from a nickel plating bath,

(d) solid state diffusion by Joule effect of the electroplated layersobtained in the steps (a), (b) and (c), washing steps being providedafter each of the electroplating operations performed in the steps (a),(b) and (c).

Preferably the electroplating step (a) of copper and lead is carried outemploying a pyrophosphate alloy bath of a new composition within thelimits set forth in the following table 1, wherein the pertinentoperative parameters are also reported.

                  TABLE 1                                                         ______________________________________                                        Composition of the alloy bath for the simultaneous                            electrodeposition of copper and lead and the pertinent                        operative conditions                                                          ______________________________________                                        Composition of the bath                                                                       concentration (moles/liter)                                   ionic species     maximum    minimum                                          ______________________________________                                        Cu(P.sub.2 O.sub.7).sub.2.sup.6-                                                                0,7        0,3                                              Pb(P.sub.2 O.sub.7).sub.2.sup.6-                                                                10 · 10.sup.-3                                                                  0,25 · 10.sup.-3                        P.sub.2 O.sub.7.sup.4-                                                                          0,2        0,1                                              HP.sub.2 O.sub.7.sup.3-                                                                         0,2        0,1                                              NH.sub.3          0,1        0                                                NO.sub.3.sup.-    0,1        0                                                Cu(HP.sub.2 O.sub.7).sub.2.sup.5-, CuP.sub.2 O.sub.7.sup.2-,                                    trace amounts                                               H.sub.2 P.sub.2 O.sub.7.sup.2-, Pb(P.sub.2 O.sub.7).sup.2-                    ______________________________________                                        Operative conditions                                                          t = 50 ± 2° C.; pH = 8.8; Cu/P.sub.2 O.sub.7 ratio = 1/7 by         weight;                                                                       current density = 10-12 A/dm.sup.2 ; countercurrent stirring.                 ______________________________________                                    

A typical composition example according to the present invention, whichcomposition has been used in many experimental tests, is the followingone: K₄ P₂ O₇.3H₂ O, 360 g/l; Cu₂ P₂ O₇.5H₂ O, 90 g/l; lead ion, 50-300ppm; H₄ P₂ O₇, q.s. to pH=8.8 (at 25° C.); NH₃, 3 g/l; NO₃ ⁻, 6 g/l.

The amount of lead contained in the coating depends: (a) on the amountof the Pb(P₂ O₇)₂ ⁶⁻ ion contained in the bath; (b) on the currentdensity, as can be observed from the results of a series of experimentaltests reported in FIG. 1 of the enclosed drawings.

In FIG. 1, the ordinates show the concentrations of metal ion in thecoating as a function of the concentrations of the ion in the galvanicbath as the abscissas (g/Kg), for four different values of the currentdensity (A/dm²), i.e., 8.6 (1); 13.2 (2); 16.1 (3); 19.3 (4). Thus itcan be clearly seen that the concentration of metallic lead in thecoating is proportional to the concentration of the ion in the galvanicbath and it decreases on increasing the current density.

Accordingly, by establishing the value of the operative current density,it is quite easy to obtain, by interpolation of the curves plotted inFIG. 1, the required value of the Pb(P₂ O₇)₂ ⁶⁻ concentration thatresults in a coating having a definite percentage of such metal.

As regards the anodic process, the anodes employed are made up ofelectrolytic copper. Since in the case of the simultaneouselectrodeposit (co-electrodeposit) of copper and lead the anodic currentyield could be higher than the cathodic current yield referred tocopper, an undesired increase in the concentration of the cupric ion canbe avoided employing a parallel-connected electrolysis tank wherein thedischarge of the excess copper is performed between inert metalelectrodes. The concentration of the lead ion may be monitored andsuitably restored through the addition of the most suitable salts; inall experimental tests the nitrate salt was employed (the nitrate ion,which performs the function of a depolarizing agent, undergoes aconstant consumption due to cathodic reduction; lead nitrate additionsmake up fully or partially for such consumption).

As regards the zinc electrodeposit (step b), an acid bath containingsulfuric acid is preferably employed, which bath has the compositionreported in Table 2, wherein the pertinent operative parameters are alsoshown.

                  TABLE 2                                                         ______________________________________                                        Zinc plating bath                                                                         Concentration (moles/liter)                                       Ionic species maximum    minimum                                              ______________________________________                                        Zn.sup.2+     2.0        0.7                                                  SO.sub.4.sup.2-                                                                             2.5        1.0                                                  ______________________________________                                        Operative conditions                                                          t = 20-40° C.; pH = 3.0 ± 0.5; current density 10-20 A/dm.sup.2     countercurrent stirring.                                                      ______________________________________                                    

Anodic and cathodic current yields are close to 100%. Anodes are made upof 99.9% zinc.

The electrodeposit of nickel (step c) is carried out preferably from anacid bath having the composition shown in Table 3, wherein the pertinentoperative conditions are also put into evidence.

                  TABLE 3                                                         ______________________________________                                        Zinc plating bath                                                                           Concentration (moles/liter)                                     Chemical species                                                                              maximum    minimum                                            ______________________________________                                        Ni.sup.2+       1.5        0.7                                                SO.sub.4.sup.2- 1.3        0.5                                                Cl.sup.-        0.5        0.1                                                H.sub.3 BO.sub.3                                                                              0.6        0.3                                                ______________________________________                                        Operative conditions                                                          t = 30-60° C.; pH = 3.5 ± 0.5; current density 2-10 A/dm.sup.2      countercurrent stirring.                                                      ______________________________________                                    

Anodes are made up in each case of 99.9% pure nickel foil.

A specific example of the nickel plating bath according to the inventionis as follows: NiSO₄.7H₂ O, 330 g/l; NiCl₂.6H₂ O, 45 g/l; H₃ BO₃, 38g/l; pH=3.5; t=50±1° C.; (specific conductivity: 70 mS at 50° C.).

As already mentioned above, the various layers obtained in the galvanicelectroplating operations disclosed previously are subjected, after afinal washing with cold water, to a reciprocal diffusion by Joule effect(d), by applying to the wire a suitable voltage value, for instance bymeans of three stainless steel rolls; thus a first heating step isrealized and afterwards a second soaking step is performed; the ratiobetween the length of the first section and that of the second one is ofabout 1:2.

The values of voltage and current density required for a correctdiffusion process depend:

(1) on the wire diameter; (2) on the thickness of the coating; (3) onthe running speed of the wire; (4) on the absolute and relative valuesof the lengths of the heating and soaking sections. Such values for thecoatings according to the present invention disclosed in the followingare not much different from those usually employed for the coating ofsteel wires with Cu and Zn. However, they are to be determinedempirically by checking, at a steady-state condition, the obtainment ofthe temperature value that ensures a complete homogenization of thecoating, through the observation of the coating colour which becomesbright yellow in the absence of nickel, whereas at increasing nickelconcentrations it takes on lighter tones (till the silver white tone).For indicative purposes, the following values are reported which wereobtained in pilot plant experimental tests and which were optimal:

A=27.5±0.5; V=2.1±0.1 at running speeds of 2 m/min.; wire diameters 1.4mm; coating of 4.2 g/Kg, containing 3% Ni.

Coatings so obtained have an average composition between the followingminimum and maximum values:

Cu: 59-70%

Zn: 30-33%

Ni: 0.75-8%

Pb: 0.2-2%

and, in the case of application to steel wires under the operativeconditions mentioned above, they give rise to final products having thecharacteristic features of the present invention.

The following Table 4 shows for exemplification purposes the results ina number of experimental tests performed on steel wires of 1.3 mmdiameter coated with a minimum amount of 3.7 g/Kg and a maximum amountof 4.4 g/Kg of the alloy of the average composition mentioned above.Such coating shows rubber adhesion values always comparable to those ofa 69/31 brass coating (such coating being considered as a reference andbeing obtained under the same operative conditions); at nickelconcentrations between 0.75 and 3.0% the values of the adhesion strengthto rubber show a sharp improvement.

                                      TABLE 4                                     __________________________________________________________________________    Analytical results obtained with a 1.4 mm diameter wire                                       alloy                                                         Test                                                                             % composition of the alloy                                                                 amount                                                                            CR CRS       Adhesion                                     No.                                                                              Cu Zn  Ni Pb g/kg                                                                              Kg Kg/mm.sup.2                                                                        ΔCRS                                                                         Kg σ                                   __________________________________________________________________________    0  68.8                                                                             31.2                                                                              -- -- 3.76                                                                              167                                                                              111.6                                                                              -10.8                                                                              56.5                                                                              9.5                                      1  65.8                                                                             32.6                                                                               0.75                                                                            0.85                                                                             3.91                                                                              181                                                                              120  -2.4 64.6                                                                             16.2                                      2  65.0                                                                             32.0                                                                              2.1                                                                              0.90                                                                             3.87                                                                              185                                                                              125.5                                                                              +3.1 71.6                                                                              7.7                                      3  64.1                                                                             32.0                                                                              3.0                                                                              0.90                                                                             4.05                                                                              188                                                                              123.7                                                                              +1.3 62.6                                                                             10.0                                      4  64.7                                                                             30.8                                                                              3.6                                                                              0.90                                                                             4.04                                                                              189                                                                              124  +1.6 55.6                                                                              7.2                                      5  62.6                                                                             32.0                                                                              4.5                                                                              0.90                                                                             4.16                                                                              189                                                                              124  +1.6 53.2                                                                             11.2                                      6  61.2                                                                             31.5                                                                              6.4                                                                              0.90                                                                             4.23                                                                              189                                                                              123.8                                                                              +1.4 59.4                                                                             11.7                                      7  60.0                                                                             32.1                                                                              7.0                                                                              0.90                                                                             4.19                                                                              189                                                                              124.3                                                                              +1.9 49.2                                                                              9.8                                      8  59.0                                                                             32.6                                                                              7.6                                                                              0.90                                                                             4.40                                                                              189                                                                              122.9                                                                              +0.5 51.5                                                                             10.2                                      __________________________________________________________________________     Copper and nickel determined by AA, 2 analytical determinations for each      one; g/kg 2 determinations; CR: ultimate tensile stress; CRS: specific        ultimate tensile stress; CRS reference 122.4; adhesion 15 determinations,     1/2 inch immersion (about 1.25 cm), time 35', temperature 155 ±            τ° C. Rubber mix: the standard mix; σ: standard              deviation.                                                               

The analysis of the surface of the sample No. 3 carried out by XPSspectrometry shows the lead signal at 144 eV (E_(bond)). Such signal isalso present after removal of a 400 Å layer by bombardment with 5 keVAr⁺, as can be observed in FIG. 2 of the enclosed drawings, wherein theabscissas show the bond energies (E_(bond)) in eV and the ordinates showthe number of electrons (N.sub.(E)).

A scanning Auger spectrometry analysis of samples 3 and 7 gave thefollowing results:

Sample 3--Sample 3 consists of an alloy (4.05 g/kg) containing 3% nickeland 0.90% lead. Carbon and oxygen are present on the surface due toatmospheric pollution, and trace amounts of chlorine and sulfur are alsopresent. The surface of the sample is well coated. The compositionprofile in the first 0.2 μm thickness layer puts into evidence aremarkable homogeneity with respect to Cu and Zn (O and C only arepresent on the surface and their signals disappear after about onehundred Å); iron is present just in trace amounts at the detectabilitylimit.

Sample 7--The chemical analysis of the coating gives the followingaverage values of nickel and lead concentrations: Ni, 7% of the totalamount and Pb 0.90% of the total amount.

The investigations of the state of the surface of sample 7 carried outby Secondary Electron Micrography (SEM) at 500× and 2000× shows thepresence of stains which are irregular both as regards their shapes andas regards their depths, as well as of longitudinal striations.

A point-to-point analysis of the first kind of stains (which are on theother hand very rare) showed that the same consisted of surface scalesof carbon materials, as the carbon (C 1s) and oxygen (O 1s) signals onlywere shown to be present in the same. On the contrary, a point-to-pointanalysis of the nearby zones showed that the coating is made up to Cu,Zn, Ni and Pb.

A compositional map was made of an area comprising one of the observedlongitudinal striations. Such analysis was performed for:

(a): copper (peak at 919 eV), (b): nickel (peak at 713 ev), (c): zinc(peak at 992 eV); the analysis showed that copper is absent from saidstriations whereas only nickel is present therein.

On the contrary zinc and lead are spread almost homogeneously everywhereboth inside and outside said longitudinal striations.

Accordingly, it can be concluded that the surface coating of sample No.7 does not consist of a quaternary alloy but that a full separation ofnickel occurs, which nickel has formed an alloy with zinc but not withcopper.

The decrease in the adhesion already remarked in Table 4 for sample 7can be presumably ascribed to such fact, which occurs for a nickelpercentage of 7% or higher. This is confirmed by the adhesion resultrelating to sample 8 (Ni: 7.6%), which also is lower than the value ofthe standard sample.

Moreover, a compositional profile of sample No. 7 carried out by Augerspectrometry, starting from the coating surface and going towards thecoating steel interface showed the presence of significative amounts ofoxygen down to about one thousand Å from the surface. The compositionalprofile of oxygen follows very closely the compositional profile of zincso that the presence can be suspected of oxide traces also within themass of the coating itself.

It is to be observed that the steel wire coated with an alloy amountbetween 2 and 8 g/kg in the presence of 0.75-7% Ni and 0.2-2% Pb can bevery easily drawn.

The wire, examined under the electron microscope or by scanning Augerspectrometry after drawing shows a surface homogeneously coated on whichquite a negligible number of defects are present.

Analytical tests and adhesion tests carried out on a standard cord madeup of four wires of 0.25 mm diameter (average thickness of the coating0.3 μm) gave results which are reported in Table 5.

                                      TABLE 5                                     __________________________________________________________________________    Wire cords of steel coated with Cu/Zn/Ni/Pb                                   __________________________________________________________________________    Analyses and adhesion strengths                                               Chemical analysis                                                             Test                                                                             1.4 mm diam. wire  Cord 2 + 2 × 0.25                                 No.                                                                              g/kg                                                                             Cu %                                                                              Zn %                                                                              Ni %                                                                              Pb %                                                                              g/Kg                                                                             Cu %                                                                              Zn %                                                                              Ni %                                                                              Pb %                                     __________________________________________________________________________    STD                                                                              4.05                                                                             62.5                                                                              37.5                                                                              --  --  3.32                                                                             69.2                                                                              30.8                                                                              --  --                                        9 4.34                                                                             61.0                                                                              34.2                                                                              3.9 0.9 3.92                                                                             65.5                                                                              31.4                                                                              2.1 1.0                                      10 4.55                                                                             62.3                                                                              32.0                                                                              4.9 0.8 3.46                                                                             63.7                                                                              32.4                                                                              2.8 1.1                                      11 4.49                                                                             64.1                                                                              33.4                                                                              1.6 0.9 3.67                                                                             63.3                                                                              32.4                                                                              3.2 1.1                                      12 4.37                                                                             64.1                                                                              32.5                                                                              2.4 0.9 3.48                                                                             64.3                                                                              32.4                                                                              2.2 1.1                                      13 4.50                                                                             62.4                                                                              33.8                                                                              2.9 0.9 3.64                                                                             64.4                                                                              33.2                                                                              1.3 1.1                                      __________________________________________________________________________    specific ultimate tensile                                                                       adhesion                                                    Test                                                                              stress        as obtained (1)                                                                          after aging (2)                                  No. CRS    ΔCRS                                                                           Kg  σ                                                                          Δ                                                                           Kg  σ                                                                           Δ                                  __________________________________________________________________________    STD 122.4  --     45.6                                                                              3.5                                                                              --  23.1                                                                              3.5 --                                       9   120.1  -2.3   48  4.0                                                                              +2.4                                                                              24  2   +0.9                                     __________________________________________________________________________     (1) Average over 18 determinations of two different vulcanizations:           reference: cords 2 + 2 × 0.25 with a 69/35 brass coating, 4.2 g/kg      (rubber mix: the standard mix, 1/2 inch immersion (about 1.25 cm),            vulcanization 155 ± 1° C. for 35 minutes).                          (2) treated with steam for 12 hours on 10% NaCl solution at 120° C                                                                              

Samples examined had a percentage amount of lead close to 1% and apercentage amount of nickel between 1.3 and 4.0%. All cord samples showafter vulcanization (the standard mix was employed according to the ASTMstandard procedure for such tests) a value of the adhesion strengthsthat is sharply higher than the reference sample value; such improvementin the adhesion strength in some instances is higher than 10%. Moreover,after steam treatment under pressure of a 10% NaCl solution at 120° C.for 4 hours, the adhesion loss is always much lower with respect to thestandard sample with a gain in the case of sample No. 11, containing3.2% Ni and 1% lead, of about 20%.

The present invention has been disclosed with particular reference tosome of its specific embodiments but it is to be understood thatmodifications and changes can be introduced in the same by those who areskilled in the art without departing from its true spirit and scope.

We claim:
 1. A process for the production of electroplated steel wires,characterized by the following series of operations:(a) simultaneouslyelectrodepositing copper and lead from an alloy bath onto a steel wire,(b) electrodepositing zinc from a zinc plating bath, (c)electrodepositing nickel from a nickel plating bath, (d) solid statediffusion by Joule effect of the layers obtained by the electroplatingsteps (a), (b), and (c), a washing step being provided after each one ofthe electroplating operations (a), (b) and (c).
 2. A process for theproduction of electroplated steel wires according to claim 1, whereinsaid simultaneous electrodeposit of copper and lead of step (a) iscarried out employing a pyrophosphate alloy bath of the followingcomposition:Cu(P₂ O₇)₂ ⁶⁻ : 0.3-0.7 moles/l Pb(P₂ O₇)₂ ⁶⁻ : 0.25×10⁻³-10×10³ moles/l P₂ O₇ : 0.1-0.2 moles/l HP₂ O₇ ³⁻ : 0.1-0.2 moles/l NH₃: 0-0.1 moles/l NO₃ ⁻ : 0-0.1 moles/l Cu(HP₂ O₇)₂ ⁵⁻, CuP₂ O₇ ²⁻, H₂ P₂O₇ ⁻², Pb(P₂ O₇)²⁻ trace amounts;Cu/P₂ O₇ ratio=1/7 by weight; pH=8.8;the bath being kept under the following operative conditions:temperature=50±2° C. current density=10-20 A/dm².
 3. A process for theproduction of electroplated steel wires according to claims 1 or 2,wherein said zinc electrodeposit step (b) is carried out employing anacid bath of the following composition:Zn²⁺ : 0.7-2.0 moles/l SO₄ ²⁻ :1.0-2.5 moles/lsaid bath being kept under the following operativecontions: temperature=20°-40° C. current density=10-20 A/dm²countercurrent stirring.
 4. A process for the production ofelectroplated steel wires according to claims 1 or 2, wherein saidnickel electrodeposit of step (c) is carried out from an acid bath ofthe following compositon:Ni²⁺ : 0.7-1.5 moles/l SO₄ ²⁻ : 0.5-1.3 moles/lCl⁻ : 0.1-0.5 moles/l H₃ BO₃ : 0.3-0.6 moles/l pH: 3.5±0.5said bathbeing kept under the following operative conditions: temperature=30°-60°C. current density=2-10 A/dm² countercurrent stirring.
 5. Apyrophosphate alloy bath for electrodepositing copper and lead on steel,said bath being characterized in that it has the followingcomposition:Cu(P₂ O₇)₂ ⁶⁻ : 0.3-0.7 moles/l Pb(P₂ O₇)₂ ⁶⁻ : 0.25×10⁻³-10×10⁻³ moles/l P₂ O₇ : 0.1-0.2 moles/l HP₂ O₇ ³⁻ : 0.1-0.2 moles/l NH₃: 0-0.1 moles/l NO₃ ⁻ : 0-0.1 moles/l Cu(HP₂ O₇)₂ ⁵⁻, CuP₂ O₇ ²⁻, H₂ P₂O₇ ²⁻, Pb(P₂ O₇)²⁻ trace amounts; Cu/P₂ O₇ ratio=1/7 by weight; pH=8.8.6. Coated steel wires obtained by the process claimed in any one ofclaims 1 or 2, said wires being characterized by an alloy coatingcontaining copper, zinc, nickel and lead, of the following averagecomposition:Cu: 59-70% Zn: 30-33% Ni: 0.75-8% Pb: 0.2-2%.
 7. A processfor the production of electroplated steel wires according to claim 3,wherein said nickel electrodeposit of step (c) is carried out from anacid bath of the following composition:Ni²⁺ : 0.7-1.5 moles/l SO₄ ²⁻ :0.5.1.3 moles/l Cl⁻ : 0.1-0.5 moles/l H₃ BO₃ : 0.3-0.6 moles/l pH:3.5±0.5said bath being kept under the following operative conditions:temperature=30°-60° C. current density=2-10 A/dm² countercurrentstirring.
 8. Coating steel wires obtained by the process claimed inclaim 3, said wires being characterized by an alloy coating containingcopper, zinc, nickel and lead, of the following average composition:Cu:59-70% Zn: 30-33% Ni: 0.75-8% Pb: 0.2-2%.
 9. Coated steel wires obtainedby the process claimed in claim 4, said wires being characterized by analloy coating containing copper, zinc, nickel and lead, of the followingaverage composition:Cu: 59-70% Zn: 30-33% Ni: 0.75-8% Pb: 0.2-2%.